Integrated circuits (IC) require electrically conducting connections to provide an electrically conductive connection between the IC's contact pads to the IC package leads or terminations. This is often achieved with the use of bonding wires. Some examples of bonding wire materials include gold, copper, and aluminum. Gold bonding wire is corrosion resistant, have a homogeneous chemical composition and stable mechanical properties. Gold bonding wire is used in plastic packages because it is faster and easier to use. Copper bonding wire, however, is an alternative to gold bonding wire due to copper's conductivity, material properties, and economical benefit. Copper wire of smaller diameter can achieve the same performance as gold bonding wire of a bigger diameter.
Challenges, however, exist in bonding the copper bonding wire to the aluminum contact pads. Specifically, the formation of a copper-aluminum (Cu—Al) intermetallic compound at a bonding interface between the copper bonding wire (i.e. copper bonding ball) and the aluminum pad has led to failures at the bonding interface. For example, the Cu—Al intermetallic compound has poor mechanical properties (e.g., brittle, easily cracks). In addition, galvanic corrosion (i.e., ion diffusion) occurs at the bonding interface, which deteriorates the electrical and mechanical properties of the compound thereby leading to failures at the bonding interface.
One option to address corrosion is to coat the copper bonding wire with palladium. The distribution of palladium, however, on the interface surface of the copper bonding ball is dependent on electronic flame off (EFO) current during the ball bonding process, which leads to inconsistent distribution results. Thus, a majority of the palladium can form around the neck of the copper bonding ball and not on the interface surface. As a result, the inconsistency of distributing the palladium on the copper bonding ball interface results in a copper-aluminum interface as above, which leads to galvanic corrosion.
The same challenges exist for solder ball applications used on IC's and printed circuit boards (PCB). In some compositions, solder also contains copper along with tin and/or silver. Thus, ion diffusion also occurs between the elements in the solder composition and the aluminum contact, which deteriorates the electrical and mechanical properties of the compound thereby leading to failures at the bonding interface.